Fix flickering on Android
This is particularly noticeable in debug builds on WebGL pages, but also appears during startup when loading normal pages. requestRender() causes Android to swap buffers under the assumption that the buffer contains a fully rendered frame, but when calling it from `doFrame` there are no guarantees that Servo has finished compositing yet. This causes stale buffers to be composited instead, leading to flickering at startup when there is no content, or general jerkiness on pages using animation callbacks as previous frames replace current frames.
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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix#21796
- [x] These changes do not require tests because no android integration tests.
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Fix Android back button behavior
Pressing the back button should call mServoView.goBack if possible.
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- [ ] `./mach build -d` does not report any errors
- [ ] `./mach test-tidy` does not report any errors
- [X] These changes fix#21401 (github issue number if applicable).
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- Add sysroot path to environment variable 'CPPFLAGS':
When checking C preprocessor, the 'configure' script of libbacktrace uses 'CPPFLAGS' rather than 'CFLAGS' and doesn't get the correct search path. (#15758)
This check passes on Linux because the '/lib/cpp' fallback is available there.
- Introduce CMake toolchain file for Android cross compiling:
CMake needs several variables [1] to cross compile for Android.
It works (accidentally) on Linux because cmake-rs sets compilers correctly and binutils for Linux & Android are pretty much the same.
[1] https://cmake.org/cmake/help/v3.7/manual/cmake-toolchains.7.html#cross-compiling-for-android-with-the-ndk
